1. Field of the Invention
The present invention relates to a conical portion growth control method which grows a conical portion of a single crystal rod by pulling up the crystal rod from a melt by using the Czochralski method and to an apparatus for use in the control method.
2. Description of the Related Art
In this kind of conical portion growth control method, a single crystal is grown by soaking a seed crystal having a diameter of approximately 10 mm in a melt and pulling up the seed crystal from the melt. In other words, after dislocations are eliminated by shortening the growing diameter of the seed crystal to approximately 3 mm, the diameter of the crystal is increased so as to grow a desired cylindrical body portion.
Since the diameter increasing portion (conical portion) of the crystal is not used as a product, it is necessary to make the length as small as possible so as to reduce costs of producing the single crystal. However, if the conical portion is too short, that is, if the crystal diameter is rapidly increased, the atoms of the crystal are disarranged and the body portion cannot be grown as single crystal.
The crystal diameter is increased by lowering the temperature related to the melt or by reducing the crystal pull-up speed. Although response in the crystal diameter to the change in the crystal pull-up speed is even prompter than response in the crystal diameter to the change in the temperature related to the melt, the atoms of the crystal are likely to be disarranged in the case where the change in the crystal pull-up speed is used to control the diameter. On the other hand, if a desired shape of the conical portion is set, the crystal diameter is measured and electricity to be supplied to a heater for heating the melt is controlled so as to obtain the desired shape, a large amount of hunting occurs and thus the surface of the crystal is made uneven. It is necessary to prevent the atoms of the crystal from being disarranged by making the slope of the desired shape gentler than a slope ideal for making the conical portion as short as possible. In short, the conical portion is longer than necessary.
Conventionally, a target temperature pattern in which the temperature of a melt falls with the passage of time is set, and the amount of electricity supplied to a heater for heating the melt is controlled so that the temperature related to the melt reaches the desired temperature. Since the amount of hunting is small in this control method, the surface of the crystal is not so uneven.
However, since the shape of the conical portion also depends upon the diameter of a narrow portion of the crystal, a target diameter of the body portion, the crystal pull-up speed, the rotation speed of a pulling shaft, the rotation speed of a crucible, the vertical position of the crucible with respect to the heater, the inner diameter of the crucible, the amount of a melt in the crucible and so on, it is difficult to obtain the shape of a conical portion having high reproducibility. Therefore, in order to prevent the disarranging of the atoms of the crystal, it is necessary to make the slope of a target temperature pattern with respect to the elapsed time gentler than an ideal slope of the temperature pattern for making the conical portion as short as possible. As a result, the conical portion is longer than necessary.